To do a little expectation-setting, the narrowband MDS of a modern, not that spectacular, UHF RX chain of better than -140dBm is not that unusual. So when I talk about a sensitive RX chain, I’m talking about those sorts of levels.
In that kind of context, -24dBm seems deafeningly loud, and -80dBm seems kind of normal. One must of course make adjustments for PSD and decide what constitutes “narrowband”. Sent from my iPhone > On Jan 4, 2021, at 11:49 PM, Marcus D Leech <patchvonbr...@gmail.com> wrote: > > If the TX and RX frequencies are a few 100kHz different then what you want > is a duplexor arrangement where the TX frequency is strongly attenuated ahead > of the RX. > > This is how repeaters work when multicoipled to a single antenna. The TX > frequency is often attenuated 90dB in the RX path. > > > > Sent from my iPhone > >> On Jan 4, 2021, at 11:33 PM, Lukas Haase <lukasha...@gmx.at> wrote: >> >> Hi Marcus, >> >>>> On 01/04/2021 12:25 PM, Lukas Haase wrote: >>> In an RFID system, the RX is *designed* to be up-close-and-snugly with >>> the other ends TX. There's NO reason to have an input >>> stage that is sensitive in the usual RF sense. So, completely >>> different engineering goals than in a typical RF system. >> >> That's not quite true. Sensitivity *is* important (in the RF sense) because >> it defines operating distance. >> Yes, many people claim RFID systems are forward link limited (which was >> true) but state-of-the art tags can have a sensivivity of -24dBm which >> corresponds to over 20m free space, 900 MHz. >> The response from the tag at the reader is around -80dBm. The R2000 chip >> that I mentioned has a sensitivity in excess of that. >> >> Indeed, this is the reason why *sensitivity* (due to self interference) is >> the limiting factor in RFID. >> >>>> I have thought of a limiter already. This could be an option. >>>> It's true, I haven't found limiters with lower power levels. >>>> >>>> Two questions here: >>>> >>>> - How/why would they add to the noise figure? >>> Any limiter diode has shunt capacitance. Which means that the degree to >>> which input power is shunted to ground is proportional >>> to the input frequency and shunt capacitance. ANY attenuation >>> (whether resistive dissipation or shunt-to-ground pathways) in >>> front of the first gain stage adds *DIRECTLY* to the noise figure of >>> that stage. So, let's say you have a nice small-signal LNA with >>> a notional noise figure of 0.5dB, and you put 10dB of loss in front >>> of it--the noise figure now becomes 10.5dB. For RFID type applications >>> this doesn't matter that much--see my "up close and snugly" >>> comments. But for "ordinary" RF receive chains, you generally want >>> to minimize noise figure while maximizing gain and linearity. >>> >>> There are exceptions--for example at HF (below 30MHz or so), the input >>> noise is *utterly dominated* by galactic background noise and >>> atmospherics--there's no point in having an input stage with a noise >>> figure below perhaps 5-10dB. So for HF, input stages tend to >>> be optimized for linearity at higher input levels--because even >>> distant signals can be quite strong at HF--particularly on the lower end. >>> >>>> - The large self-interfere would result in clipping (hard nonlinearities). >>>> Is this any problem for the LNA (gain desensitivisation etc.) >>> Well. Yeah. That's what the P1dB parameter is all about--the input >>> level at which gain is compressed by 1dB. >>> >>> The overall take-away here is that generic radios (whether they be SDRs >>> or others) should be thought of as *components* in an >>> overall *engineered RF system*. That may mean things like relays to >>> shunt the RX pathway during TX, circulators, attenuators, >>> diplexors, filters, RF-plumbing-in-general. >> >> Yes. >> >> To clarify limiter vs non-limiter. >> My self interfering signal from TX can be up to 20-25dBm. The desired signal >> is a modulation bandwidth (few 100 kHz) away and the receiver should be as >> sensitive as possible to that signal. >> >> Now I have the two options: >> >> 1.) Adding a normal attenuator of 40dB. This ensures the USRP input is >> always less than 25-40=-15dBm but it also reduces my desired signal by 40dB. >> In other wirds, my noise figure worsens by 40dB, as you mentioned. >> >> 2.) Adding an RF limiter with flat leakage +5dBm (I found SKY16602-632LF). >> Afterwards I add a 20dB attenuator to get the max to 5-20=-15dBm. >> >> If I understand your explanation correctly, there is no real difference >> between these two (because the limiter would still crushes signal and with >> it desensitizes the desired signal on top). >> Is this correct? >> >> Would you prefer one option over the other? >> >> Thanks again, >> Lukas >> >> _______________________________________________ USRP-users mailing list USRP-users@lists.ettus.com http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
